Plate tectonics - Introducing Tectonics, Rock Structures and Mountain Belts - page 19

hot spots now lie well within the plate
interiors, as is the case with Hawaii.
On the ridges themselves, activity
is not continuous, but is concentrated
into more active and less active regions.
The best example of an active region is
Iceland (Figure 3.13A), where volcanic
activity associated with the mid-Atlantic
ridge has been so great over the last
50 million years or so that the island
has become the only part of the ridge
to be elevated above the surface.
Hot spots have been linked to the
concept of mantle plumes (Figure
3.13B), over which there has been
considerable controversy. Some have
argued that the plumes represent warm
material rising from the base of the
mantle, and provide the driving mecha-
nism for plate motion. Sophisticated
geochemical techniques have been

devised to categorise the igneous prod-
ucts of plumes, and many examples of
excessive volcanic activity recognised in
the geological record have been ascribed
to plumes on that basis. In the Earth's
circulatory system, cooled material is
returned to the mantle by means of the
subduction zones, whereas the plumes,
rather than the ridges, may be the main
vehicle for transferring fresh, warm
material to the surface. It seems likely,
however, that the relationship between
convection currents, plumes and plate
movements is a complex one for which
we do not yet have a satisfactory model.

engine: hot material produced by
radioactive decay in the deep mantle
rises to the surface and is replaced
by descending material supplied by
the cooled upper layer. Unlike our
boiling liquid analogy, Earth's convec-
tion takes place by solid-state flow,
which is very much slower than liquid
flow, but the experimental study of the
behaviour of materials demonstrates
that solid state flow can indeed take
place, given the right conditions ( see
Chapter 4). The plates, therefore, can
be viewed as the cooled surface layer
of the Earth's convection system.
Evidence from gravity surveys and
earthquake-wave data shows some
support for variations in mantle density
consistent with the existence of warmer
and cooler regions of the deep mantle.
However, the detailed geometry of these
convection currents has proved dif-
ficult to establish, although the simple
circulatory cells envisaged originally
by Holmes are clearly an over-simplifi-
cation. In particular, detailed evidence
of plate movements demonstrates that
the positions of both ridges and sub-
duction zones move laterally across
the Earth's surface through time, which
is difficult to reconcile with a simple,
static, convective-cell system. As an
example of this, consider the case of the
Antarctic plate, which is surrounded on
all sides by constructive boundaries. It
follows that the plate must grow in size
through time, and that its boundaries
must move across the Earth's surface,
implying that there cannot be a direct
connection to a long-lived uprising
hot column. A further problem arises
when we consider the positions of the
present-day hot spots, such as Hawaii,
that are located in the middle of plates.

3

18 19

What drives the plates?
As we have just seen, there is consider-
able evidence to show how the plates
move relative to each other, but why
they move is not such an easy question
to answer as might at first appear. The
early conveyor-belt model for conti-
nental drift ( see Figure 3.4) implied that
the movement of the continents was
caused by the circulation of mantle ma-
terial taking place by solid-state flow.
This model of the movement of ma-
terial within the mantle is a modern
development of the much older idea of
mantle convection currents , popular-
ised in the 1930s by Arthur Holmes.
The concept of convection is central
to understanding the processes that
govern crustal behaviour, and is familiar
to all of us from observing the boiling of
liquid in a pan heated from below. Warm
liquid expands, becomes less dense, and
rises to the surface where it is cooled;
the heated liquid is replaced by cooler
liquid, which, because it is denser,
descends to the bottom of the pan.
According to the convection model,
the Earth behaves like a giant heat

present-day
ridge axis

ridge formed in
last 4 m.y.

approximate limits of
present-day hot-spot

A

volcanic region

plate

0 200km

plume

B

200km

Figure 3.13 A. Simplified map of the Iceland hot
spot, believed to overlie a plume. Note that the
current position of the hot spot lies east of the
ridge, which has migrated westwards away from
the hot spot in the last 4 million years. Based
on Saemundsson, 1974. B. Sketch section of a
plume causing a thinning and stretching of the
lithosphere above.